We've grown it here in VT and it's very invasive; particularly in wet heavy soil.
It gets down to -20F quite often (never has hit -30) but always seem to have good snow cover.

Did you try to contain it with any kind of barrier? I have a corner in my yard that I thought I would put down that plastic 4" barrier and just plant it by itself. Would that contain it or would the roots run under it and into the lawn? Does it need deadheading too, to prevent spreading? Also, everything I read said it was gorgeous--did you find it that way or should I just pitch it and chalk it up to experience and "try" to remember to research a plant before purchasing!! Thanks for any help and recommendations you can give me!

I tried putting it a plastic container about 14"D and buried it up to the rim but it never did well. You may be able to contain it but I'm not sure 4" is deep enough. To be sure try a 10" deep barrier. I never had a problem with reseeding only via roots. Also it hasn't gotten into the lawn'.
It is a nice plant and give it a try. If it becomes a problem you should be able to get rid of it, or at least control it without too much effort.

Thanks squirejohn!